The present invention relates generally to occupant restraint systems in passenger vehicles and, more particularly, to a method for activating an occupant restraint or other impact responsive system which utilizes a single sensor, discriminates between impact events in which the device is to be activated and impact events in which the device is not to be activated, and activates the device within a desired time period.
Impact responsive devices, such as air bags and fuel shut-off controls, are rapidly becoming standard features on most passenger vehicles. A conventional air bag inflates to protect the vehicle passengers in response to an acceleration force on the vehicle. Electro-mechanical sensors are mounted at various locations in the vehicle outside the passenger compartment, such as on the frame rails and radiator, to detect the impact and activate the air bag.
In a collision, there are two types of crash pulses: one in the crush zone where the frontal frame structure absorbs energy in a crash, and the other in the occupant compartment where the undisturbed portion of the vehicle body remains rigid. The structural responses in the two zones are different. The crush zone undergoes rapid velocity change as it deforms early in the crash, and the occupant compartment experiences a rigid body deceleration with smaller deceleration magnitude and longer duration than those in the crush zone.
Most air bag sensors are of the ball and tube type. Inside each sensor, a gold-plated steel ball is held in place at the end of a short tube by a magnet. In a forward impact, the ball "breaks" free from the magnet and travels along the tube toward two electrical contacts. Crash sensors are located near the front of the vehicle and a "safing" sensor near the passenger compartment. The safing sensor is connected in series with the crash sensors. The air bag is deployed when at least one of the crash sensors and the safing sensor are activated and the activation times are overlapped. The safing sensor serves to confirm that a crash is so severe that it warrants an air bag deployment; and it also serves to prevent an air bag from inadvertent deployment in case there is an electrical short circuit in the crash sensor. However, systems employing such remote electro-mechanical sensors require multiple sensors and complex control systems.
In view of these deficiencies, vehicle designers have attempted to implement single point sensing systems which utilize a single electronic sensor, such as an accelerometer, in the passenger compartment of the vehicle. One such system is disclosed in U.S. Pat. No. 5,068,793 issued to Condne et al. The Condne et al. system utilizes a single accelerometer to generate a deceleration signal indicative of deceleration of the vehicle. The deceleration signal is then reduced by a variable deceleration threshold signal and the resulting signal is integrated over time. The variable deceleration threshold signal is a feedback signal taken from the integrated signal. When the integrated signal exceeds a triggering threshold value, an impact is detected.
Unfortunately, prior systems, such as the Condne et al. system, are based on a time domain analysis of the deceleration signal which has been found to be relatively inaccurate in discriminating between must-activate impacts and must-not-activate impacts. Further, prior systems have experienced difficulties in reacting to a must-activate impact within a desired activation time.
It is thus apparent that a need exists for a method for activating an impact responsive device which utilizes a single sensor, distinguishes between must-activate impacts and must-not-activate impacts, and activates the device within a desired time period.